1. Field of the Invention
The present invention relates to data transmission control systems which can suitably control the amount of data to be transmitted in a network system such as an ATM (asynchronous transfer mode) exchange system, on the basis of different data parameters including, for example, priority parameters of data requiring immediateness or real time communication, priority parameters of mail data less requiring the immediateness and possible variable transmission band parameters, so as to realize effective use of communication lines of the network system, and more particularly, to a data transmission control system which, even during communication after a call is set by a terminal connected to the network system, can suitably control the amount of data to be transmitted in the network.
2. Description of the Related Art
As light transmission techniques are developed these years, it has been possible to transmit a wide range of data relatively inexpensively. In order to comply with such a situation, an ATM-based communication system has been studied and examined even in the Comite Consultatif International Telegraphique et Telephonique (CCITT). The ATM communication system refers to one where, substantially on the same principle as a high-speed packet system, data is divided into packets of a short fixed length called `cells` and the cells are asynchronously transmitted for data exchange.
Accordingly, in the ATM communication system, there may occur such a situation that a cell is discarded due to temporary traffic variations. In order to avoid such a situation, in an exchange system based on the ATM communication system, a method for determining at the time of call setting the number of cells of the call to be transmitted has been suggested. In this cell-number determining method, consideration is paid to the fact that cells are statistically multiplexed so that parameters for each call are statistically calculated to grasp the call acceptable by the exchange system.
However, even when the cell-number determining method is employed for the call setting, since the quality of each call must be maintained, cells are not always transmitted on actual communication lines and thus the communication lines are set with a certain degree of allowance.
For example, even when such calls having relatively loose discard quality as voice, e.g., calls having a discard rate of about 10.sup.-3 are multiplexed on a communication line having a relatively narrow band of 1.5 Mbps, the capacity of the communication line having a sufficient allowance is set, which results in that only 80% of cells can be multiplexed on the line. Thus, in this case, it cannot be that the line is efficiently used.
For the purpose of realizing efficient line use, it is considered to multiplex calls irrespective of discard quality such as mail transmission, i.e., calls having high discard priority levels on the line, to thereby realize more efficient use of the line. In the case of such calls as mail transmission, however, the call band is usually set to be somewhat large. For this reason, when there are many calls having ordinary quality, concentrated transmission of cells of calls having low discard quality involves such a problem that the respective traffics of the cells of such calls are mutually canceled with the result that the calls having low discard quality are extremely reduced in data passage rate.
Thus, in order to effectively multiplex calls having no time transparency and allowing a certain degree of cell discard on the line without fixedly determining the band, it is necessary during call continuation to somehow modify the data transmission format, e.g., to modify the data transmission band of the terminal.
However, it is impossible in the current call setting procedure to modify the data transmission format. This will be explained by referring to FIG. 10 showing the call setting procedure carried out in ordinary communication. That is, as shown in FIG. 10, in the current communication, the data transmission format, e.g., data transmission band, required by a user terminal at the time of call setting is continued until the call is completed, thus making it impossible to change the transmission format of the data.
Accordingly, when such current call setting procedure as mentioned above is employed, it is impossible during the call continuation to change the data transmission format of the user terminal in accordance with the operating state of the network. Thus, even when a terminal having a low call priority and requiring no immediateness is present in the exchange system, it is impossible to realize finely-controlled and flexible data transmission with effective use of the line.
In this way, such a transmission system which performs data multiplexing in statistic manner as the ATM communication system has been defective in that, during continuation of a call transmitted from a user terminal, it is difficult to attain fine control over the data transmission of the user terminal according to the call discard quality, call use band, the variability of the use band, etc., and thus it is difficult to realize effective use of lines.